1. Field of the Invention
The invention relates to a communications system with transmission parameters configurable dependent upon a reliability value and to a method for the adaptation of these transmission parameters on the basis of reliability information, which is calculated for every bit in a datastream by means of an (also iterative) estimation algorithm.
2. Related Technology
Radio communications systems transmit information via transmission channels, of which the properties can generally only be described through statistical values or statistical models. In addition to the noise, such as the additive white noise (additive white Gaussian noise; AWGN), the multi-path propagation with the associated fading also plays a significant role in the interpretation and performance of radio communications systems. In the case of multi-path propagation, the radio signals broadcast from the antenna reach different propagation paths of the reception system. The radio signals can be reflected, fragmented or scattered by obstacles and accordingly reach the reception unit with an arbitrary amplitude and arbitrary phase. Within the reception unit, the radio signals of all propagation paths are superimposed and lead to constructive or destructive superposition (interference), wherein the constructive interference brings about a mutual amplification of signal components, and the destructive interference brings about a mutual cancellation of signal components. Moreover, in the case of a movement of the participants, Doppler effects cause additional interfering influences on the radio signal.
In order to achieve an optimal data transmission with minimal use of energy and/or maximum range, especially over a time-variable transmission channel, the data to be transmitted are secured within the transmission unit by means of a channel coding, which systematically adds redundancy to the actual information. In the reception unit, the systematically added redundancy allows an improved detection of the originally transmitted data. The channel coding means that the effects of the interfering influences named above on the transmitted data can be reduced within the reception unit, so that the data reconstruction is more reliable as a whole.
To achieve the most efficient possible exploitation of a time variable transmission channel, the data transmission rate should also be time variable and matched in an adaptive manner to the current properties of the transmission channel. Accordingly, for example, with good transmission channels, a high-quality digital modulation method or a relatively high code rate can be adjusted. By contrast, with poor transmission channels, it should be possible to select relatively more robust transmission modes (modulation method with reduced symbol size or respectively smaller code rate).
The evaluation of the current channel properties for the adjustment of the transmission parameters is achieved according to the prior art by an estimation of the signal-to-noise ratio (signal-to-noise ratio, SNR) and the received signal strength (received signal strength indication RSSI) or the evaluation of transmission errors (packet error rate, PER). An estimation of the SNR can be obtained, for example, via the comparison of known synchronization or training sequences with the received signal. Since the channel influences, such as the noise or the amplitude and phase distribution of the multiple paths is arbitrary and can therefore be described only with statistical models, and the estimation is implemented only over a short period, the estimation is not optimal and provides estimation inaccuracies. Statements or estimates with low estimation errors are possible only via lengthy averaging. Instead of known synchronization and training sequences, the estimated received symbols can also be used after the detection for the estimation of the SNR (decision feedback method). However, in this case, detection errors in the data and their probability of occurrence have a considerable influence on the accuracy of the estimation algorithm for the SNR.
An estimator for the SNR provides further disadvantages. The SNR does not reflect the influence of interference on the performance of the decoder for the channel decoding. In spite of a good SNR, it is possible that the interference and the channel properties lead to the channel decoding delivering poor results. Conversely, in spite of a relatively poor SNR, an error-free decoding may still be possible. One further disadvantage occurs if the signal quality during a data transmission is to be determined continuously. If synchronization and training sequences are to be used for the estimation, these must be present in the signal continuously, in order to be able to implement a new estimation at an arbitrary timing point. A continuous estimation can only be realized with decision feedback algorithms (decision feedback algorithms) with the disadvantages named above.
German patent specification DE 196 51 593 A1 relates to an arrangement, which is used for the optimization of a data transmission via a bidirectional radio channel. For the bidirectional radio channel, various modulation types can be selected at the transmitter end, wherein the code rate of the forward error correction (FEC) and the transmitter power can be adjusted simultaneously. At the receiver end, modules are provided, which determine the error rate and communicate this back to the transmitter. On the basis of the returned error rate, parameters such as the modulation type, the code rate in the channel coding, the transmitter power and the size of the data packets are configured or adjusted within the transmitter unit in such a manner that a defined error rate cannot be exceeded in the receiver.
The disadvantage with this arrangement is that only the error rate, which occurs with the transmission via the radio channel, is used as the sole criterion for the configuration of the transmission unit. Accordingly, it is very improbable that the transmission unit is configured in an optimal manner, and that a maximal data transmission rate with error-free transmission is possible with this arrangement. This method is based upon the evaluation of transmission errors; the adaptation of the parameters before the occurrence of errors is not possible.